CHARGE-COUPLED-DEVICE SPECTRA OF THE GALILEAN SATELLITES - MOLECULAR-OXYGEN ON GANYMEDE

We have obtained 3200-7800 Angstrom CCD spectra of the Galilean satell ites at a variety of orbital longitudes, with spectral resolution betw een 3 and 18 Angstrom and signal-to-oise ratios of up to 2000 at 6000 Angstrom. Despite the higher resolution and signal-to-noise ratio than previous published spectra, no new features are seen on Io, Europa, a nd Callisto. However, Ganymede shows an unusual and previously unrepor ted shallow absorption feature at 5773 Angstrom, which is much stronge r on the trailing side, and a weaker band at 6275 Angstrom. The featur es are apparently due to diatomic oxygen, and require simultaneous ele ctronic transitions in two adjacent molecules: they are the strongest visible-wavelength absorption bands in solid or liquid oxygen. Because condensed pure oxygen is not stable at Ganymede surface temperatures and pressures, the oxygen must be trapped in other surface materials, perhaps in water ice, with the constraint that O-2 molecules must be c lose enough together for simultaneous electronic transitions. Magnetos pheric bombardment of water ice is a plausible production mechanism fo r the O-2. Continuum shapes on all four satellites are more clearly se en than in previous spectra. The shape of the continuum is very simila r on Ganymede and Callisto but is strikingly different on Europa, indi cating a different origin for the nonice component on Europa. The shap e of the Europa continuum suggests that allotropes or compounds of sul fur may be the dominant spectrally active materials in the visible spe ctrum. No convincing changes since 1978 are visible in Io's spectrum, despite the high resurfacing rates, suggesting that volcanic resurfaci ng tends to overpaint areas with more material of the same composition .